EFI return line, use emissions return?
............... to claridy, I don't have any Vacuum in my tank at this time, the open cannister return line allone will see to that.)
Here's the thing.......
I was assuming you have the original pressure/ vacuum filler cap, which, as I said, has a "pop off" pressure both for outgoing pressure and ingoing ---vacuum
The return line / canister system has a certain amount of static pressure, even though it may be "open" so it DOES cause SOME vacuum IE "negative pressure" in the tank, as the fuel is pulled out of it by the pump
The fact that for now, your pump(s) are both up front means that at least the low pressure pump IS operating UNDER a vacuum on the suction side of the pump. Again, this is due to the pressure vacuum cap, and the restriction in the canister system, (which may also have limiting valves)
Even the older cars with an "open vent" operate with the suction side of the mechanical pump "under a vacuum" which PROMOTES fuel vapor/ boiling.
That is, ANY time you lower the pressure on a liquid, you LOWER the boiling point of that liquid. So here you have the original supply line from the tank being "sucked on" by the pump up front, which lowers the boiling point IN THAT LINE. Additionally, the line comes up front past the exhaust and hot engine, which promotes boiling/ vapor even more.
And of course, last, you have the recirculation problem.
